In industrial plants, low-voltage loads such as, e.g. controllers, amplifiers and the like, are supplied with a direct voltage of preferably 24 Volts, which is harmless to persons. Suitable power supply devices which provide such a direct voltage can supply output currents of 20 A and more. At such high currents, protection devices such as, e.g. fuses or circuit breakers, must be connected in series with the respective loads in order to protect these and, in particular, the feedlines, against thermal overload and short-circuit currents. In order to be able to trip circuit breakers reliably magnetically in the event of an electrical fault, for example a short circuit, tripping currents are required which amount to about 7.5-times the nominal current specified with regard to the power supply device. The tripping characteristic of circuit breakers is the result of their time/current tripping characteristic such as, for example, the class-B characteristic. With the usual dimensioning, such high tripping currents for circuit breakers can be supplied in the event of a short circuit by traditional 50 Hz transformers used as power supply devices.
Due to high electrical losses and the great weight, such 50 Hz transformers are more and more frequently replaced by electronic power supply devices such as, e.g. switched-mode power supplies and transformer power supplies clocked at high frequency in industrial power supplies. However, electronic power supply devices usually is limit the output current very rapidly when an electrical fault occurs, that is to say within between 10 and 100 μsec, to 1.1- to 1.5-times the value of the nominal current in order to protect loads and feedlines against thermal overloads and short-circuit currents. Although electronic power supply devices are on the market which can also generate an output current of up to 2.5-times the nominal current for a short time, that is to say up to 4 seconds, these low currents are not adequate for reliably and safely tripping circuit breakers magnetically.